A higher torque is required of the drive transmission mechanism for transmitting a drive force to first and second output shafts rotating integrally with the first and second screws of a biaxial extruder. In order to realize the higher torque, counter-measures have been exemplified by raising the power of the motor, by increasing the bearing size for the increase in the radial force which is generated at the meshing portions of gears such as first and second output gears fitted individually on the first and second output shafts, or by enlarging the width of teeth to ensure the tooth bearing. Because of the extremely restricted axial distance between the two shafts, however, troubles occur in case the bearing size is to be increased or in case the tooth width of gears is to be enlarged, thus making it difficult to transmit the drive force of high torque.
This transmission of the drive force of high torque is known to have been achieved by a drive transmission mechanism disclosed in Japanese Patent Laid- Open No. 1185/1987. This drive transmission mechanism 33 for a biaxial extruder of the prior art will be described with reference to FIGS. 5 and 6. Of these, FIG. 5 is a diagram showing a drive line, and FIG. 6 is a side elevation showing the gear arrangement of FIG. 5. Incidentally, rectangles and thick lines appearing in FIG. 5 indicate gears and shafts, respectively.
This drive transmission mechanism 33 is constructed to include: first and second output gears 3 and 4 fitted with an axial displacement respectively on first and second output shafts which are connected to the first and second screws of the biaxial extruder (the reference numerals 1 and 2 generally refer to both the output shafts and the screws) first lower and upper gears 5 and 6 arranged respectively below and above the axis of the first output gear 3, as shown in FIG. 6; second lower and upper gears 7 and 8 arranged respectively below and above the axis of the second output gear 4, as shown in FIG. 6; and a reduction gear train D for quartering the output of a common drive source C equally and distributing the quarters to those first lower and upper gears 5 and 6 and second lower and upper gears 7 and 8.
The reduction gear train D is composed of: gears 24 and 25 meshing with the first upper and lower gears 6 and 5, respectively; gears 22 and 23 rotating integrally with the gears 24 and 25, respectively; gears 29 and 30 meshing with the second upper and lower gears 8 and 7, respectively; gears 27 and 28 rotating integrally with the gears 29 and 30, respectively; a center shaft 32 equipped at its one end with a gear 21 meshing commonly with the gears 22 and 23 and at its other end with a gear 26 meshing commonly with the gears 27 and 28; and a drive gear 31 fitted on a middle portion of the center shaft 32 for transmitting the drive force of the common drive source C. This reduction gear train D provides a gear train having a symmetric structure with respect to the first and second screws. Incidentally, the center shaft 32 is constructed of toothed joint shafts 32a and 32b for correcting both the phase shift due to the torsional rigidity caused by the difference between the lengths of the output shafts 1 and 2 and the phase shift at the time of assembly.
In the drive transmission mechanism 33 for the biaxial extruder thus constructed according to the prior art, the drive force from the common drive source C is transmitted to the center shaft 32 through the drive gear 31. This drive force is halved and distributed by the center shaft 32 to the first screw line and the second screw line. At this time, the toothed joint shafts 32a and 32b composing the center shaft 32 have their diameters and lengths determined to correct the phase shifts due to the torsional rigidity of the output shafts 1 and 2. Moreover, the drive forces thus distributed are quartered from the gears 21 and 26 at the two ends of the center shaft 32 to the gears 22 and 23 and the gears 27 and 28 and are distributed to the first vertical and second vertical screw lines. The drive forces thus distributed to the first and second vertical screw lines are transmitted vertically equally to the first upper and lower gears 6 and 5 and the second upper and lower gears 8 and 7 by the gears 24 and 25 and the gears 29 and 30, which rotate integrally with the gears 22 and 23 and the gears 27 and 28, respectively. As shown in FIG. 6, moreover, the drive forces are transmitted to the first and second output shafts 1 and 2 to rotate the first and second screws such that the radial loads upon the first and second output shafts 3 and 4 are offset by interposing the first output gear 3 vertically between the first upper and lower gears 6 and 5 and by interposing the second output gear 4 vertically between the second upper and lower gears 8 and 7.
Thus, in the drive transmission mechanism 33 for the biaxial extruder of the prior art, the phase shift due to the torsional rigidity cased by the difference between the lengths of the output shafts 1 and 2 is corrected to synchronize the first and second screws by suitably setting the diameters and lengths of the toothed joint shafts 32a and 32b composing the center shaft 32. Moreover, the radial loads upon the first and second output gears 3 and 4 can be offset to transmit the drive force of high torque to the first and second screws by interposing the first output gear 3 vertically between the first upper and lower gears 6 and 6 and by interposing the second output gear 4 vertically between the second upper and lower gears 8 and 7 to transmit the drive power.
In order to synchronize the first and second screws of the biaxial extruder, the drive transmission mechanism 33 having the aforementioned structure of the prior art has its toothed joint shafts 32a and 32b determined so suitably in their diameters and lengths as to correct the phase shifts due to the torsional rigidity of the first and second output shafts. After these determinations, therefore, in the reduction gear train D having a structure symmetric with respect to the first and second screw lines, the distribution of the drive force at the gear meshing portions has to be equalized so that the drive force may be equally transmitted to the two lines.
In the drive transmission mechanism 33 for the biaxial extruder according to the prior art, however, the first half distribution of the drive force from the common drive source C to the first and second screw lines is accomplished by the center shaft 32 of the reduction gear train D. Hence, the vertical quarter distribution to the upper and lower gears of the first screw line and the vertical quarter distribution to the upper and lower gears of the second screw lines have to be accomplished separately of each other so that as many as eleven gears are required in the shown example. Thus, the drive transmission mechanism 33 of the prior art has problems that the structure is enlarged, that the number of portions requiring the phase adjustments of gears is increased to trouble the assembling work, and that the accumulation of the phase shifts, even if slight, between the gears will fail to equalize the distribution of the drive power.
For the phase adjustments at the assembling time, moreover, only one tooth number of the gears at the two ends of each of the toothed joint shafts 32a and 32b is made different from the other so that the fitting positions of the joint shafts 32a and 32b may be determined to absorb the phase difference. This phase difference absorption at the assembling time is carried out, as disclosed in Japanese Patent Publication No. 12415/1987, by using a spline shaft having its two ends splined with only one tooth number being made different. However, this phase difference absorption is accompanied by measuring the phase shift of one gear train with reference to that of the other and re-assembling the toothed joint shaft and the spline shaft in the positions capable of absorbing the phase difference. Thus, this proposal has a problem that the adjustment takes a long time.
It is, therefore, an object of the present invention to provide a drive transmission mechanism for a biaxial extruder, which can be easily constructed and assembled with the minimum number of gear trains and which can distribute the drive force equally at the meshing portions of gears.
Another object of the present invention is to provide a drive transmission mechanism for a biaxial extruder, which can be timely adjusted to transmit the drive force equally to individual lines without any trouble of disassembling the drive transmission mechanism so as to adjust the phase difference at the assembling time or re-assembling the same.